It has been proposed, and is being tested and implemented in heart failure diagnostics to use features of the cardiogenic impedance (CI) signal for estimating contractility. Specifically, it has been proposed to use trended changes in CI-derived contractility to contribute to diagnosis and prediction of worsening heart failure events. Moreover, it may be desirable to determine pacing sites or timing intervals based on CI-derived contractility, to diagnose ischemic and/or arrhythmic events based on acute or sub acute (i.e. seconds-to-minutes time frame) changes in CI, or to guide or activity responsive changes to programmed parameters based on CI-derived contractility, among other applications.
Heretofore, it has been difficult to estimate quantities related to contractility. For instance, contractility may be used to determine information related to the strength or speed of contraction of an isolated myocyte, or alternately to local, regional, or global myocardial pumping function. Further, the term contractility has been used in scientific literature to refer to pulse pressure, rate of pressure rise, peak generated pressure, stroke volume, systolic or pre-ejection time intervals, and even synchrony, all as indicators of “contractility.”
However, a number of non-heart failure (non-HF) factors contribute to the various measurable values from which contractility is derived. These non-HF factors include preload, afterload, metabolic state, heart rate, neurohormonal influences, and the like. While these non-HF factors do exert real influence on cardiac performance, it is important to separate the effects of non-HF factors from fundamental changes to the myocardium that are to be measured. For example, it is desirable to use contractility to identify myocardium changes due to worsening heart failure, due to optimized (or non-optimized) pacing, and the like. Therefore, methods and systems are needed that provide a true and independent assessment of contractility, unaffected by non-HF factors.
This invention describes a set of methods using various sensors/measurements available on the implanted device in conjunction with CI to remove confounding factors from contractility estimates, with specific examples given as to how the methods can be applied to a heart failure trending diagnostic.